Local area networks were originally intended to interconnect the computing resources in a firm (local computer networks) and they have afterwords been adapted to allow voice transmission in addition to data transmission.
Nowadays local area networks are rapidly evolving as there is a trend towards an integration of all communications services in a firm into a single network.
The resulting network must accomodate not only conventional voice and data services, but also new services such as high resolution graphics services for CAD (computer-aided design) or CAM (computer aided manufacturing) applications, video conferencing, remote control of manufacturing, high resolution video broadcasting.
Local networks ought to have very high capacity of the order of some hundreds Mbit/s or even of the Gbit/s to be able to handle the traffic originated by a very high number of sources (some thousands); besides, source requirements may be very different as to:
transmission rate (and consequently band occupation) which may range from some kbit/s (speech services) to several Mbit/s (graphic or video services) and may be variable for a given service; PA1 maximum acceptable delay ranging from few ms (services requiring real-time information processing, such as graphics facilities or remote control of manufacturing) to some hours (e.g. data files transfer, electronic mail, etc.); PA1 error rate; PA1 amount of traffic originated by the individual sources; PA1 switching modalities: circuit switching for communications with continuity and/or synchronism characteristics or packet switchig for the other communications.
As telecommunications, and more particularly office automation are growing quickly, a local communication system ought to adapt itself easily to variations in the offered facilities, and to grow as the user requirements increase. Besides, these systems are generally private systems and their connections to the future integrated services public network ought to be simple and cost-effective.
Integrated services networks are known in which processing requirements of information originated by very different sources are met by organizing the network activity into repetitive frames, composed of two regions assigned to circuit-switched communications (basically time division multiplexed, so that each is allotted a time interval or channel of duration proportional to the transmission rate) and to packet switched communications respectively. These frames are generally referred to as hybrid frames.
An example is described in Italian Patent Application No. 67736-A/82 filed on 9th June 1982, in the name of the Applicant, which discloses a distributed variable-band switching system for speech and data intended for a topology with "islands" with high terminal concentration connected to a unidirectional folded bus. In this system a distributed bus access management is disclosed such that, for data communications the bus capacity is shared among the various islands according to an ordered-access protocol whereas the activity of the terminals inside an island can be controlled by a random-access protocol. For synchronous communications, constant-duration channels are provided which occur periodically for the whole call duration.
The solution described in the above-mentioned Patent Application is mainly studied for conventional low-rate speech /data services and hence cannot be satisfactorily used for the new applications: in effect contention accesses and collision possibility cause very low effciency, taking into account the high transmission rates envisaged. In addition, the known system requires memorization of the activity on the bus on a frame-by-frame basis and therefore it does not allow dynamic management of services whose band requirements vary during the connection (e.g. TASI or DSI systems, variable bit-rate coding) or, in general, dynamic frame management.
An example of a system with dynamic hybrid frame management is described in the paper entitled: "A hybrid switching system with variable bit rate" by G. Fioretto, L. Gabrielli, N. Lotito, M. Sposini, CSELT Rapporti Tecnici, vol. XII, No. 1, February 1984, pages 63-73.
The solution proposed in this paper is satisfactory for a switching node, but cannot be used for a local area network for constructive reasons and owing to frame management modalities. In fact the node comprises a plurality of modules interconnected by a parallel bus, which can be usefully used over short distances (some tens meters), and not over distances of some hundreds meters of of kilometers as can be necessary in a local network; in fact for these distances a parallel bus has high installation costs and, taking into account high transmission rates, presents risks of different transmission delays on the different wires. Besides, frame control is completely distributed, i.e. the generation of the "start of frame" and "region boundary" flags is allotted frame by frame to the module processing the first and the last channel of the circuit region, respectively: this entails a considerable efficiency limitation, as, owing to the long distances considered, the propagation delays of such signals and hence the waiting times necessary for a module to recognize whether or not it is entrusted woth the emission become very long. The efficiency is further limited by pauses between adjacent channels which over the above distances have durations equal to or longer than channel durations, and by the possibility of packet collisions. Finally, the dynamic speech circuit management requires sending additional signalling with consequently increased circuitry complexity.